1) Field of the Invention
The present invention relates to an image display device with a laminated structure that includes at least an anode, a cathode, and a light emitting layer.
2) Description of the Related Art
Although liquid crystal display devices require a backlight, organic electroluminescent (EL) display devices do not require a backlight. For this reason, the organic EL display devices are ideal for thinner display devices. Moreover, the organic EL display devices do not have a limitation on the angle of visibility. For these reasons, the organic EL display devices are expected to be the image display devices of the next generation.
An organic EL display device includes an organic EL element that has at least a light emitting layer between two electrodes. Voltage is applied between the electrodes, so that the light emitting layer emits light to display an image. As the organic EL element, there is know a top emission-type organic EL element in which one of the two electrodes is made of a metal such as aluminum, and the other electrode is a semitransparent electrode of LiF/AgMg, or the like. In the top emission-type organic EL element, light emitted from the light emitting layer is transmitted through the semitransparent electrode.
However, depending on the incident angle of the light emitted from the light emitting layer, some of the light is reflected at interfaces between the layers. In such a case, only a part of the light emitted from the light emitting layer is transmitted to the outside. As a result, most of the light emitted from the light emitting layer is contained within the device and cannot be extracted, resulting in poor light extraction efficiency.
“Applied Physics Letters (Vol. 78, pp. 544-546, United States, 2001)” discloses an organic EL element having higher light extraction efficiency. In the organic EL element disclosed, a high-refraction layer, that is, a layer having higher refractive index than the laminated layer in contact with the high-refraction layer, is provided on a side of the light emitting layer from where light is emitted (hereinafter, “light emission side”). FIG. 15 is the laminated structure of a conventional organic EL element 100. The conventional organic EL element 100 includes a substrate 111, an anode layer 112 made of a metal such as Al, a buffer layer 113, a hole transporting layer 114, a light emitting layer 115 that also serves as an electron transporting layer, and a cathode layer 116 made of transparent film such as ITO film. The anode layer 112, the buffer layer 113, the hole transporting layer 114, the light emitting layer 115, and the cathode layer 116 rest on the substrate 111. This organic EL element 100 further includes a capping layer 117 on the cathode layer 116, that is, on the light emission side of the light emitting layer 115. The light emitted from the light emitting layer 115 is passes to the outside via the cathode layer 116 and the capping layer 117. Some part of the light gets reflected at the anode layer 112 and then passes to the outside via the cathode layer 116 and the capping layer 117.
The capping layer 117 is the high-refraction layer. In other words, the capping layer 117 has higher refractive index than the light emitting layer 115, which is in contact with the capping layer 117, and the cathode layer 116. Light is totally reflected when it passes from a layer with a high refractive index to a layer with a low refractive index at an angle equal to or greater than the critical angle. On the other hand, light that is incident on a layer with a high refractive index from a layer with a low refractive index is not totally reflected even if the incident angle is great, and at least part of the light can enter the layer with a high refractive index. Therefore, the light that is incident on the capping layer 117 with a higher refractive index from the cathode layer 116 with a lower refractive index is not totally reflected by the interface between the cathode layer 116 and the capping layer 117, and at least part of the light can be transmitted to the outside through the capping layer 117. Thus, the amount of light that is totally reflected by the interface between the capping layer 117 and the cathode layer 116 can be reduced.
FIG. 16 is a graph to explain how the light extraction efficiency varies with the thickness of the capping layer 117. It is assumed here that the light emitting layer 115 emits red light. The “light extraction efficiency” represents a converted value of the ratio of the luminance of light emitted in the vertical direction from the organic EL element 100 to the luminance of light within the light emitting layer 115 where the same input energy strength is applied, with the luminosity factor determined by the naked eye being taken into consideration. The “luminance” is a value obtained by multiplying the radiant intensity at each wavelength by the relative luminosity factor, and then integrating the product with the wavelength. When the thickness of the capping layer 117 is 80 nanometer (nm), the light extraction efficiency, 1.43, is maximum. The thickness of the capping layer 117 is adjusted so that the organic EL element 100 can have a light extraction efficiency of 1.40 or higher.
Thus, extraction efficiency can be improved in the conventional organic EL display device. However, an increase in reflectance cannot be prevented to maintain a reasonable luminosity factor, moreover, the contrast degrades. This problem is described in greater detail, with reference to FIGS. 17 through 20.
FIG. 17 is a graph for explaining wavelength dependency of the light extraction efficiency and the reflectance of each of organic EL elements that respectively emit red (R), green (G), and blue (B) light. Curve Lb represents the light extraction efficiency of the organic EL element that emits blue light. The curve Lg represents the light extraction efficiency of the organic EL element that emits green light. The curve Lr represents the light extraction efficiency of the organic EL element that emits red light. The curve Rb represents the reflectance of the organic EL element that emits the blue light. The curve Rg represents the reflectance of the organic EL element that emits the green light. The curve Rr represents the reflectance of the organic EL element that emits the red light. The light extraction efficiency shown in FIG. 17 is the rate of the luminance of light transmitted to the outside of the organic EL element, to the luminance of light emitted from the light emitting layer. Each reflectance shown in FIG. 17 is the rate of the luminance of light returned to the outside of the organic EL element, to the luminance of light entering from the outside.
As shown in FIG. 17, the light extraction efficiency of the organic EL element of each color is higher in the vicinity of the emission peak, and the reflectance is lower than in the other wavelength regions. For example, as indicated by the curve Lr and the curve Rr, the organic EL element that emits the red light exhibits a high light extraction efficiency and low reflectance in the wavelength region of 600 nm to 650 nm, which is the wavelength range of red light. The same applies to the organic EL element that emits the blue light and the organic EL element that emits the green light. The reflectance of each organic EL element becomes higher outside the emission peak region. For example, the organic EL element that emits the blue light and the organic EL element that emits the red light each exhibit high reflectance in the wavelength region of 520 nm to 580 nm, which is shown as a “region a” in FIG. 17. The light that belongs to the high-reflectance “region a” returns to the outside of the organic EL element 100 at a higher rate. As indicated by the transmission path A2 in FIG. 18, the light that belongs to the high-reflectance “region a” is reflected at the interface between the anode layer 112 and the buffer layer 113, and then returns to the outside of the organic EL element 100 through the capping layer 117.
FIG. 19 is a graph of relative luminosity factor with respect to wavelength. Luminosity factor, which represents the eye sensitivity to light, vary with wavelengths, and is maximum at 555 nm. A relative luminosity factor is a relative value, with the luminosity factor at 555 nm being the reference value. The light that has a wavelength that falls into the wavelength range a, exhibits a relative luminosity value of 0.8 or higher as shown in FIG. 19, and is easy to recognize with the naked eye. Accordingly, in the organic EL elements of red and blue, the light that belongs to the high-reflectance “region a” is easy to recognize with the naked eye.
Especially in the organic EL elements that emit red and blue lights, it is considered that the light of the “region a”, which is returned to the outside of the organic EL element 100, is recognized with the naked eye as light having a higher luminance than it actually has. FIG. 20 is a graph for explaining the dependency of the reflectance on the thickness of the capping layer 117. The reflectance is obtained by multiplying the reflectance shown in FIG. 17 and the relative luminosity factor, and then integrating the product with the wavelength. As shown in FIG. 20, with the luminosity factor being taken into consideration, the reflectance is as high as 0.62 when the thickness of the capping layer 117 is 80 nm, with which the light extraction efficiency becomes the highest. Although not shown, the same applies to the organic EL element that emits blue light.
As described above, in the conventional organic EL element 100, the reflectance of the light outside the emission peak region is high, even where the reflectance of light in the emission peak region is low. Especially in organic EL elements that emit red or blue light, the reflectance is high in the “region a” that exhibits a high luminosity factor. Accordingly, in such organic EL elements, reflected external light is strongly visible to the naked eye. As a result, the reflected external light is added to the light on the display screen of the organic EL element 100, which results into degradation of contrast of an image to be displayed.